The modern athletic shoe is highly refined combination of many elements which have specific functions, all of which must work together for the support and protection of the foot during an athletic event. A shoe is divided into two general parts, an upper and a sole.
The upper is designed to snugly and comfortably enclose the foot. Typically, it will have several layers including a weather- and wear-resistant outer layer of leather or synthetic material such as nylon, and a soft, padded inner liner for foot comfort. Current uppers typically have an intermediate layer of a synthetic foam material. The three layers of the upper may be fastened together by stitching, gluing, or a combination of these. In areas of maximum wear or stress, reinforcements of leather and/or plastic are attached to the upper. Examples of such reinforcements are leather toe sections attached over synthetic inner layers of the toe area and heel counters made of an inner layer of plastic and an outer layer of leather.
The other major portion of an athletic shoe is the sole. Designed to withstand many miles of running, it must have an extremely durable bottom surface to contact the ground. However, since such contact may be made with considerable force, protection of the foot and leg demands that the sole also perform a shock-absorbing function. It therefore typically includes a resilient, energy-absorbent material as a midsole in addition to the durable lower surface. This is particularly true for training or jogging shoes designed to be used over long distances and over a long period of time.
The normal motion of the foot of a typical runner during running proceeds as follows. The foot hits the ground heel first, then rolls forwardly and inwardly, (abducts, everts and dorsilflexes) over the ball of the foot and the toes. As the foot rolls forward, the toes make contact with the ground; the heel leaves the ground; the toes push off from the ground; and finally the entire foot leaves the ground to begin another cycle. During the time that the foot is moving from heel strike toward ball contact, it typically is rolling from the outside or lateral side, to the inside or medial side, a process called pronation. During motion through ball and toe contact the foot rotates outward (adducts, inverts and plantarflexes) and becomes rigid as the toes prepare to push off, a process called supination. While the foot is airborne and preparing for another cycle, the foot remains supinated.
Pronation, the inward roll of the foot in contact with the ground, although normal, can be a potential source of foot and leg injury, particularly if it is excessive. Various devices incorporated either onto the upper or into the sole have been devised to limit pronation to a reasonable range. In the design of an overall sole, lateral motion control; i.e., the control of pronation and supination, must be taken into consideration. Particular care must be taken in the design of a cushioning midsole because of its inherent tendency to compress, and thus add additional lateral motion to the foot. Thus, while a cushioning midsole must be compressible to perform its shock-absorbing function, adequate lateral control for the overal shoe must still be present. While a midsole contributes to a loss of lateral control, other devices, such as heel counters or reinforcements, can be added to increase lateral control. However, control which can be added by means of such devices is limited. Therefore, a midsole cannot be designed with such compressibility that would make adequate lateral control unattainable.
Another limiting factor in the design of a cushioned midsole is the range of suitable cushioning materials. Current commercial cushioned midsoles use elastomeric foam, such as ethlene vinyl acetate EVA foam, within a narrow mid-range of hardness, or an elastomeric foam within which a gas-filled membrane is encapsulated. The use of elastomeric foam material by itself is limited to foams of relatively higher density and hardness, because low density and hardness foams are too soft and bottom out too quickly, i.e., collapse to a point where it no longer functions as shock absorber under relatively low force, and also because low hardness foams provide very little lateral stability. Hence, prior art commercial midsoles have generally been limited to higher density, relatively hard foams; i.e., foams with densities of 0.4 and above and hardness within the range of Shore A 25 and harder. The commercial use of foams within this narrow range of hardness reaches a compromise between cushioning and stability. The use of a softer foam would provide additional cushioning at a sacrifice to lateral stability. Conversely, the use of harder foams would enhance lateral stability at a sacrifice to cushioning.
The use of a membrane partitioned into a plurality of chambers which are filled with a gas, which in turn is incorporated into a foam midsole, improved the cushioning capability of the midsole over that of conventional EVA foam because it does not bottom out as rapidly. The present invention, as will be discussed more fully hereinafter, improves the cushioning capabilities of a midsole layer even further.
Other cushioning techniques have been disclosed for both athletic and dress shoes in the patent literature. For example, U.S. Pat. Nos. 2,437,227, 2,721,400 and 4,267,648 disclose the use of coil springs within a cushioning midsole layer. In the '227 and '400 patents, the cushioning midsole layers are used in dress shoes and additionally use other cushioning material such as sponge rubber. In the '648 patent, spring mechanisms, such as disc or Bellville washer-type springs, are disclosed for use in athletic shoes.
U.S. Pat. No. 4,283,864 discloses a cushioning material construction formed of integral plastic modules. The modules are composed of a plurality of levers and spaced bearing means which are incorporated into the midsole area of footwear.